Coating a piece of steel and a piece comprising zirconium

ABSTRACT

A HYDROGEN RESISTANT JUNCTION BETWEEN PIECES OF ZIRCONIUM OR ZIRCONIUM ALLOYS AND PIECES OF STEEL AND A METHOD OF MAKING SAME. THE EXPOSED SURFACES ADJACENT THE JUNCTION ARE COATED WITH AN ALLOY CONSISTING ESSENTIALLY OF AL, SI, FE, ZR AND ALLOYING METALS. THE COMPATIBILITY OF THE ALLOY WITH BOTH THE ZIRCONIUM OR ZIRCONIUM ALLOY PIECE AND THE STEEL PIECE ENABLES THE COATING TO BE CONTINUOUS AND ADHERENT TO BOTH.

July 27,.-1'971 M. ARMAND ETAL 3,595,686

COATING A PIECE OF STEEL AND A PIECE COMPRISING ZIRCONIUM Filed nec. 19, 196e INVENTORS Marcel Armand Michel Clhurveriot Jeon-Pierre Givord B Y Daw @www @Mmmm ATHEIR ATTORNEYS United States Patent fticel 3,595,686 COATING A PIECE F STEEL AND A PIECE AClVlIPlRISlliJG ZIRCUNllUll/i Marcel Armand, Michel Charveriat, and Jean-Pierre Givord, Albertville, France, assignors to Ugine Kuhlmann, Paris, France Filed Dec. 19, 1968, Ser. No. 785,175 Int. Cl. B44d 1/06 U.S. Cl. 117-71 2 Claims ABSTRACT 0F DISCLOSURE A hydrogen resistant junction between pieces of zirconium or zirconium alloys and pieces of steel and a method of making same. The exposed surfaces adjacent the junction are coated with an alloy consisting essentially of Al, Si, Fe, Zr and alloying metals. The compatibility of the alloy with both the zirconium or zirconium alloy piece and the steel piece enables the coating to be continuous and adherent to both.

Zirconium and zirconium alloys (hereafter referred to zirconium comprising metals or zirconium metals) are subject to hydrogen embrittlement at elevated temperatures. Embrittlement occurs when these metals are used in contact with a source of hydrogen, for example, when used as high pressure tubes for conducting organic liquids such as terphenyl at temperatures in the range of 300- 400 C. The zirconium comprising metals can be protected from hydrogen either by a layer of zirconium oxide formed on the surface of the metal or still better by coating with an alloy having an aluminum base.

The problem becomes complicated when a piece of zirconium metal is joined to a piece of steel, for example, where tubes of the two metals are joined. Since there is no protective coating at the zirconium metal-steel interface, hydrogen which readily penetrates steel, enters the steel in the vicinity of the junction and passes through the zirconium metal-steel interface thus attacking the zirconium metal near the junction long before attacking the remainder of the zirconium metal piece. It is then possible to have a failure in the juncture zone.

Therefore, for reasons of safety, it is necessary that the hydrogen impermeable coating must be continued beyond the zirconium metal piece for a given length on the steel piece. In this way, the hydrogen will have to travel a substantial distance in the steel before penetrating the zirconium metal and will, therefore, have a lower concentration at the junction reducing the danger of hydrogen embrittlement.

There are two known processes for protecting zirconium comprising metals against hydrogen by coating with an alloy having an aluminum and silicon base. The first process (disclosed in French Pat. No. 1,456,754) consists of immersing a piece of zirconium through a ux into an alloy of Al-Si-Zr of suitable composition at a temperature ranging between 575 and l000 C. thus forming a protective layer of a ternary compound. An improvement on that process is to reinforce those coatings by brazing a thin layer of aluminum on the alloy coat (see Belgian Pat. No. 685,799 of Aug. 22, 1966). The second process for protecting zirconium and its alloys against hydrogen with a coating of an aluminum and a silicon base consists of solid state bonding an Al-Si layer to the zirconium by diffusion under pressure but at a temperature below the melting point of the alloy. Coatings according to these methods are rich in aluminum and have the proper thickness thereby inhibiting the penetration of hydrogen into the zirconium. Between the aluminum rich coating and the zirconium metal base there is a stable and continuous intermetallic layer with the formula AlXSiyZrZ. This layer provides a barrier to the subsequent penetration of aluminum into the zirconium metal base at temperatures in the range of 100-500" C.

Unfortunately, the coatings applied by the processes dcscribed above cannot be used for protecting a piece of .zirconium metal joined to a piece of steel. In fact, if the immersing technique was used the steel extension would be rapidly dissolved in the Al-Si-Zr alloy. Coatings applied according to the method of solid state bonding under pressure would react rapidly with the steel but become separated therefrom after cooling as a result of the rupturing of the intermetallic layer formed.

It is an object of this invention to provide a hydrogen resistant junction between a piece of zirconium metal and a piece of steel. It is another object to provide a process for protecting pieces of zirconium metal near a junction with a steel piece from hydrogen embrittlement. It is a further object to provide a method of joining a piece of zirconium metal to a piece of steel.

This invention consists of making a junction between two pieces, and thereafter treating the exposed surfaces adjacent the junction, for example, by soaking, with a molten aluminum base alloy at a temperature, for eX- ample, between 575 and 800 C. The aluminum base alloy consists essentially of Al, l to 13%, by weight, Si, 0.2 to 10% Fe, 0.2 to 5% Zr and up to 15% of metal selected from the alloying elements, if any, of the steel and the zirconium. According to a preferred embodiment of this process, an additional protective layer of aluminum or aluminum alloy is thereafter secured, for example, by brazing, on at least the surfaces of the two pieces that have been treated and preferably also on a portion of the surface of zirconium which has not been treated.

The process according to this invention provides a continuous coating over the surface of the two metals that is substantially impervious to hydrogen. The coating consists of an outer layer having a composition the same as the aluminium alloy. Between the outer layer and the zirconium metal piece or the steel piece there is formed a layer of intermetallic compounds substantially comprising Al, Si and Zr over the zirconium piece and Al, Si and Fe over the steel piece. This continuous coating extending over the junction zone makes it necessary for the hydrogen which will penetrate the steel at an unprotected point on the surface to travel an extended distance through the steel before reaching the zirconium metal. By then its concentration has diminished suiiiciently to eliminate the danger of hydrogen embrittlement.

The additional coating provided according to the preferred embodiment of this invention should preferably be between 0.5 and 2 mm. thick. This additional coating provides reinforcement. This additional coating may be secured to the interior of a tube near a junction by cold drawing a thin tube Within the junction and then heating it under vacuum wln'le forcing it with an expandable mandrel against the outer tube. The original protection film serves as a brazing alloy and] enables the adherence of the additional coating to both the steel piece and the zirconium pieces near the junction. It should be pointed out that the above-mentioned intermetallic layer is not altered by the additional coating. The additional coating may be made to adhere to the zirconium metal piece outside the junction zone without the use of an intermediate brazing alloy.

The figure is a cross-section through a junction between a tubular steel piece and a tubular zirconium metal piece.

According to one mode of practicing this invention especially where it would be inconvenient to immerse the entire length of the assembled zirconium metal and steel pieces, a junction member is first prepared comprising a steel section and a zirconium metal section. Then the junction is treated with the aluminum alloy. Thereafter,

the corresponding ends of the junction member are welded to the long steel and zirconium tubes. According to one procedure, the junction section is first welded to the zirconium metal tube. Then an additional protection coat or layer is secured within the junction zone extending beyond the weld into the zirconium metal piece. Finally, the steel tube is welded to the junction member. FIG. 1 is a schematic section of a junction according to this particular mode. The junction member 1 comprises a piece 2 of Zircaloy 2 tubing approximately 400 mm. long and a piece 3 of non-alloyed steel tubing also approximately 400 mm. long. The junction 4 between the two pieces of tubing may be secured together mechanically as by threading. This junction was then soaked in an aluminum alloy bath containing 7% VSi by weight, 3% Zr and 5% Fe which formed a rst coating 5 on the internal surface of the junction member. At both ends of the junction membe'r the coating was pickled to eliminate contamination of the Welds. At point 6 the junction member was welded to the zirconium alloy tube 7. Thereupon a thin tube 8 of an aluminum alloy of 1% Si and 0.2% Zr, was cold drawn onto the inner surface of the zirconium alloy tube and onto the inner surface of the coated portion steel section of the junction member. An expandable mandrel was placed within the tube and junction member and the whole assembly were heated in a vacuum to effect complete adhesion along the entire length of the tube 8. Finally at point 9 the other end of the junction member was welded to the steel tube 10.

Having thus described my invention in detail and with the particularity required by the patent laws, what is desired to have protected by Letters Patent is as follows.

We claim:

1. A process for protecting a piece of zirconium comprising metal near a junction with a steel piece from hydrogen embrittlement comprising treating the exposed surfaces to hydrogen adjacent the junction with a molten alloy consisting essentially of A1, l to 13%, by weight, Si, 0.2 to 10% Fe, 0.2 to 5% Zr and up to 15% selected from the alloying elements, if any, of the steel and Zirconium thereby providing a continuous hydrogen impervi: ous coating over the surface.

2. A process according to claim 1 wherein an additional protective coating comprising an alloy of Al is applied to the surfaces adjacent the junction.

References Cited UNITED STATES PATENTS 2,755,542 7/1956 Boegehold 117-114C 2,916,397 12/1959 Allan Chin 117-114C 3,000,756 9/1961 Logan 117-131 3,155,536 11/1964 Freeman 117--114C HYLAND BIZOT, Primary Examiner U.S. Cl. X.R. 29-194, 197 

